The present invention relates generally to thermal cutting and, more particularly, to the thermal severing of relatively thick glass plates or sheets.
The art of thermally cutting relatively thick glass sheets or plates is becoming increasingly popular in an effort to overcome the arduous, time consuming cutting techniques of the prior art, such as scoring followed by breaking and diamond saw cutting, for example. The most common method of thermally severing glass involves nicking one edge of the glass sheet to form a score therealong in alignment with the intended line of cut and then applying radiant heat along the intended line of cut from a non-contact thermal source spaced from the glass sheet surface. However, the foregoing method is not very reliable in producing true straight cuts because it is believed that the radiant heat emanating from the source converges and enters the glass surface along the intended line of cut and then diverges as it propogates inwardly through the glass thickness. The progressively widening heated area creates at the tension layer, a heat band of substantially greater lateral extent than the intended line of cut. This can cause the glass to fracture uncontrollably in a meandering path along such band. Additionally, when attempting straight cuts which are offset from the centerline of a glass blank, the cut has a tendency to run in an arcuate path or bow into the narrower severed piece to produce a curved edge, sometimes referred to as a "banana edge".
The thermal cutting method and apparatus disclosed in U.S. patent application Ser. No.677,977, filed Apr. 19, 1976, assigned to the same assignee as the present invention, provided a solution to some of these problems by employing continuous surface contact, conductive heat against the surface of the glass sheet along the entire desired line of cut. While this thermal cutting technique has admirably served the purpose for which it was intended in severing thick glass sheets, it was found that, in addition to the thermal energy developed to effect the cut, a significant portion of such energy was consumed in physically moving the heavy severed pieces apart against the resistance offered by the supporting table. Also, compressive surface stresses induced in the glass blank as a result of the irregularities inherent in the glass supporting surface of a so-called "flat" table top posed problems in repeatedly making straight dimensional cuts.